Fuel cell separator and coating method of seperator for fuel cell separator

ABSTRACT

A fuel cell separator includes a metal substrate having a surface; an ion penetration layer including carbon diffusion-inhibiting ions extending from the surface of the metal substrate into the metal substrate; and a carbon coating layer disposed on the surface of the metal substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2016-0138365 filed in the Korean IntellectualProperty Office on Oct. 24, 2016, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION (a) Field of the Invention

A method of coating a fuel cell separator and a fuel cell separator aredisclosed.

(b) Description of the Related Art

A fuel cell stack is a device discharging electricity, water, and heatby reacting hydrogen with oxygen in the air. The fuel cell stack may bedivided into a repeatedly stacked part such as an electrode layer, aseparator, a gas diffusion layer and a gasket, and a non-repeating partsuch as a locking element required to lock a stack module, an encloserprotecting the stack, an interface with a vehicle, and a high voltageconnector or the like. The fuel cell stack, however, has many inherentrisks because high voltage electricity, water and hydrogen coexist inthe same location.

A fuel cell separator—the most important part in the fuel cellstack—requires low contact resistance and low corrosion current.Conventional metal separators have excellent electrical conductivity butalso have poor corrosion characteristics, and thus have lowerdurability. When hydrogen cations generated during driving using thefuel cell directly contact the separator, corrosion may occur. Inparticular, when an uncoated metal is used, there is a greater risk ofcorrosion. Moreover, electrical conductivity may decrease due to theformation of an oxide on the surface of the uncoated metal. In addition,dissociated and eluted metal cations contaminates a Membrane ElectrodeAssembly resulting in deteriorated performance of the fuel cell. Theseproblems have conventionally been addressed by coating the separator.

Carbon in the coating material has high electrical conductivity and goodcorrosion characteristics. But when carbon is coated onto the metallayer using a plasma enhanced chemical vapor deposition (“PECVD”), thecarbon atoms penetrate into the metal separator due to the hightemperature of the process, weakening the bond between the metalsubstrate and the carbon coating layer. In this case, the weak adhesionstrength and the low density of the carbon layer causes degraded contactresistance and corrosion characteristics when driving using a fuel cell.

SUMMARY OF THE INVENTION

An example embodiment of the present disclosure provides a method ofcoating a fuel cell separator.

Another example embodiment of the present invention provides a fuel cellseparator.

An example embodiment of a method of coating a fuel cell separatorcomprises the steps of: providing a metal substrate; forming an ionpenetration layer by penetrating carbon diffusion-inhibiting ions into asurface of the metal substrate; and forming a carbon coating layer onthe ion penetration layer.

After providing the metal substrate, the method may further include thestep of removing an oxide film disposed on a surface of the metalsubstrate surface.

In forming the ion penetration layer, the carbon diffusion-inhibitingion may be a nitrogen or boron ion.

Formation of the ion penetration layer may be performed at about 300degrees Celsius to about 550 degrees Celsius for about 10 minutes toabout 120 minutes.

Formation of ion penetration layer also may be performed under anatmosphere including about 10 volume % or greater of an ion-forming gas.

Formation of the ion penetration layer also may include penetratingcarbon diffusion-inhibiting ions by plasma or ion implantation.

Formation of the carbon coating layer may be performed at about 300degrees Celsius to about 550 degrees Celsius.

Formation of the carbon coating layer may include exciting a carbon atomin a carbon precursor gas using plasma to form the carbon coating layer.

Formation of the carbon coating layer may be performed under anatmosphere including about 3 volume % to about 30 volume % of a carbonprecursor gas.

An example embodiment of a fuel cell separator according to the presentdisclosure comprises: a metal substrate; an ion penetration layerincluding carbon diffusion-inhibiting ions extending from a surface ofthe metal substrate into the metal substrate; and a carbon coating layerdisposed on the ion penetration layer.

The carbon diffusion-inhibiting ion of the ion penetration layer may beinterposed between metal atoms of the metal substrate.

The ion penetration layer may include about 5 wt % to about 30 wt % ofthe carbon diffusion-inhibiting ion.

The ion penetration layer may include about 5 wt % to about 85 wt % ofcarbon.

The carbon diffusion-inhibiting ion may include a nitrogen or boron ion.

The thickness of the ion penetration layer may range from about 30 nm toabout 300 nm.

The thickness of the carbon coating layer may range from about 10 nm toabout 1000 nm.

The density of the carbon coating layer may range from about 2.0 g/cm³to about 3.0 g/cm³.

In an example embodiment, the binding force of the carbon coating layeris enhanced due to the presence of the ion penetration layer.

In a further example embodiment, the density of the carbon coating layeris enhanced due to the presence of the ion penetration layer.

In an example embodiment, the fuel cell separator has low contactresistance and corrosion potential characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a cross-section of an exampleembodiment of a fuel cell separator.

FIG. 2 is a schematic view showing a detailed cross-section of thelayers of a fuel cell separator.

FIG. 3 is a schematic view showing a cross-sectional view of a surfaceof an example embodiment fuel cell separator having a carbon coating butno ion penetration layer.

FIG. 4 is a scanning electron microscopic (“SEM”) image showing asurface of a carbon coating layer in an example embodiment of a fuelcell separator according to Example 1.

FIG. 5 is a graph of the results from a Glow Discharge Optical EmissionSpectroscopy (“GD-OES”) test of an example embodiment of a fuel cellseparator according to Example 1.

FIG. 6 is an SEM image showing a surface of a carbon coating layer of anexample embodiment of a fuel cell separator according to ComparativeExample 1.

FIG. 7 is an optical microscopic (OM) photograph of an exampleembodiment of a fuel cell separator according to Example 1 afterevaluating adhesion strength.

FIG. 8 is an optical microscopic (OM) photograph of an exampleembodiment of a fuel cell separator according to Comparative Example 1after evaluating adhesion strength.

FIG. 9 is a graph showing the results of evaluating contact resistancefor example fuel cell separators according to Examples 1 to 4 andComparative Example.

FIG. 10 is a graph showing the results of evaluating corrosion currentdensity for example fuel cell separators according to Examples 1 to 4and Comparative Example.

DETAILED DESCRIPTION

The advantages and features of the present invention and the methods foraccomplishing the same will be apparent from the example embodimentsdescribed hereinafter with reference to the accompanying drawings.However, the present invention is not limited to the example embodimentsdescribed hereinafter, but may be embodied in many different forms. Thefollowing example embodiments are provided to make the disclosure of thepresent invention complete and to allow those skilled in the art toclearly understand the scope of the present invention, and the presentinvention is defined only by the scope of the appended claims.Throughout the specification, the same reference numerals denote thesame elements.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may also be present. In contrast, when an element is referredto as being “directly on” another element, there are no interveningelements present.

In some example embodiments, detailed descriptions of well-knowntechnologies will be omitted. Unless otherwise defined, all terms(including technical and scientific terms) used herein have the samemeaning as commonly understood by one of ordinary skill in the art. Inaddition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising”, will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements. Further, as used herein, the singularforms are intended to include the plural forms as well, unless thecontext clearly indicates otherwise.

FIG. 1 is a schematic view showing a cross-section of an exampleembodiment of a fuel cell separator.

Referring to FIG. 1, a separator 100 [need to show on FIG. 1] for anexample embodiment of a fuel cell separator comprises: a metal baselayer 10; an ion penetration layer 20 including carbondiffusion-inhibiting ions 21 that penetrate into the surface of metalsubstrate 10; and a carbon coating layer 30 disposed on ion penetrationlayer 20. The fuel cell separator 100 shown in FIG. 1 is merely anexample, but the present invention is not limited thereto.

Hereinafter, each component of fuel cell separator 100 is described.

Metal base layer 10 may be composed of a variety of materials. In oneexample embodiment, metal substrate 10 may be a stainless steel sheet.In a further example embodiment, metal substrate 10 may be an SUS300-based steel sheet.

As shown in FIG. 2, in an example embodiment, an ion penetration layer20 extends from the surface of metal substrate 10 into metal substrate10. Ion penetration layer includes carbon diffusion-inhibiting ions 21that assist in improving the binding force and the density of the carboncoating layer 30, and ultimately, the contact resistance and corrosioncharacteristics of fuel cell separator 100.

FIG. 2 is a schematic view showing a cross-section of an exampleembodiment of a fuel cell separator 100. As shown in FIG. 2, ionpenetration layer 20 includes carbon diffusion-inhibiting ions 21,wherein the carbon diffusion-inhibiting ions 21 are disposed in thespaces between the metal atoms 11 comprising metal substrate 10. Carbondiffusion-inhibiting ions 21 of Ion penetration layer 20 prevent carbonatoms disposed on the surface of metal substrate 10 (e.g. fromapplication of a carbon coating layer 30) from diffusing into the metalsubstrate 10. Because diffusion of carbon atoms 21 into metal substrate10 is prevented, the majority of the carbon atoms 31 in carbon coatinglayer 30 remain on the surface of metal substrate 10. Carbon atoms 31 onthe surface of metal substrate 10 may make carbon-carbon bonds 32 withother carbon atoms in the carbon coating layer, so the binding forcebetween the carbon coating layer 30 and the metal substrate 10 may beenhanced and the density of the carbon coating layer 30 is improved.Thus, fuel cell separator 100 may have low contact resistance andcorrosion potential characteristics.

In contrast, as shown in FIG. 3, when ion penetration layer 20 is notformed prior to application of carbon coating layer 30, the carbon atoms31 in carbon coating layer 30 diffuse into metal substrate 10 betweenmetal atoms 11. As a result, only a small amount of the carbon atoms 31in carbon coating layer 30 remain on the outermost surface of metalsubstrate 10, and the binding force and density of carbon coating layer30 are diminished.

Like this, according to an example embodiment of the present invention,by providing the ion penetration layer 20 in which the carbondiffusion-inhibiting ion 21 is interposed between metal atoms 11 of themetal substrate 10, the fuel cell separator 100 may have low contactresistance and corrosion potential characteristics. In an exampleembodiment, metal substrate 10 may be a stainless steel sheet and metalatom 11 of metal substrate may include a metal atom present in thestainless steel sheet, for example, iron (Fe), chromium (Cr), nickel(Ni), or molybdenum (Mo). As used herein, the term “interposing thecarbon diffusion-inhibiting ion 21 between metal atoms 11” means thatthe carbon diffusion-inhibiting ion 21 is penetrated or solid-solved inthe austenite face centered cubic structure (FCC) lattice.

Ion penetration layer 20 may include from about 5 wt % to about 30 wt %of carbon diffusion-inhibiting ion 21. When the amount of carbondiffusion-inhibiting ion 21 is too low, the ability of ion penetrationlayer to prevent diffusion of carbon into metal substrate 10 isinsufficient. When the amount of carbon diffusion-inhibiting ion 21 istoo high, it may be delaminated from metal substrate 10 by an externalimpact due to hardening of the surface. Thus, the fuel cell separatorhas low durability.

In addition, during formation of carbon coating layer 30, carbon atoms31 in carbon coating layer 30 may partially diffuse into ion penetrationlayer 20, so that the composition of the ion penetration layer is fromabout 5 to about 85 wt % of carbon atoms 31. When the amount of carbonatoms 31 diffusing into ion penetration layer 20 is too low, theformation of carbon-carbon bonds 32 between carbon atoms in the ionpenetration layer and carbon atoms in the carbon coating layer 30 isreduced, resulting in a lower binding force of carbon coating layer 30.When the amount of carbon atoms 31 diffusing into ion penetration layer20 is too high, the density of carbon coating layer 30 is decreased.

Carbon diffusion-inhibiting ion 21 may be any ion capable of inhibitingthe diffusion of carbon atoms 31 by being interposed between metal atoms11 in metal substrate 10. In an example embodiment, carbondiffusion-inhibition ion 21 may be any ions having an atomic number ofless than or equal to 20. In a further example embodiment, carbondiffusion-inhibiting ion 21 may be a nitrogen ion or a boron ion.

Ion penetration layer 20 may have a thickness of from about 30 nm toabout 300 nm. When ion penetration layer 20 is too thin, ion penetrationlayer 20 may be unable to sufficiently prevent diffusion of carbon atomsinto metal substrate 10. When ion penetration layer 20 is too thick, thehardness of the fuel cell separator may be excessively increased if toomuch nitrogen penetrates into ion penetration layer 20, resulting in thefuel cell separator being easily broken by an external impact.

Ion penetration layer 20 may be formed by plasma or ion implantation. Anexample method of forming ion penetration layer 20 as part of an examplemethod of coating a fuel cell separator is described in detail later.

Carbon coating layer 30 is disposed on the ion penetration layer 20.Application of carbon coating layer 30 may ensure that the fuel cellseparator has high electrical conductivity and low corrosion potential.

Carbon coating layer 30 may have a thickness of from about 10 nm toabout 1000 nm. When carbon coating layer 30 is too thin, it isinsufficient to provide the desired electrical conductivity andcorrosion characteristics. When the carbon coating layer 30 is toothick, stresses applied to the fuel cell separator may causedelamination of carbon coating layer 30 from metal substrate 10. In afurther example embodiment, the thickness of the carbon coating layer 30is from about 20 nm to about 100 nm.

According to another example embodiment, the density of carbon coatinglayer 30 may be increased due to formation of ion penetration layer 20.In a further example embodiment, the density of carbon coating layer 30may range from about 2.0 g/cm³ to about 3.0 g/cm³.

A method of coating a fuel cell separator 100 according to an exampleembodiment includes the steps of: providing a metal substrate 10;forming an ion penetration layer 20 by penetrating carbondiffusion-inhibiting ions 21 into a surface of metal substrate 10; andforming a carbon coating layer 30 on ion penetration layer 20. Each stepis described in greater detail below.

First, a metal substrate 10 is provided. The metal substrate 10 mayinclude any metal typically used for a fuel cell separator 100 withoutlimitation. In one example embodiment, metal substrate 10 may becomposed of a stainless steel sheet. In a further example embodiment,metal substrate 10 may be composed of a SUS 300-based steel sheet.

An oxide film may form on the surface of the metal substrate 10 due tocontacting with air, for example. Because the oxide film increasescontact resistance, in a further example embodiment, the method mayfurther include the step of removing an oxide film from the surface ofmetal substrate 10 prior to formation of ion penetration layer 20. Theoxide film may be removed by, for example, heating metal substrate 10and applying argon plasma.

Ion penetration layer 20 by penetrating carbon diffusion-inhibiting ions21 into the surface of the metal substrate 10. Ion penetration layer 20extends from the surface of metal substrate 10 into metal substrate 10.

Plasma or ion implantation may be used to implant carbondiffusion-preventing ions 21 into metal substrate 10. The method ofusing plasma includes applying plasma under an ion-forming gasatmosphere for providing carbon diffusion-inhibiting ions 21 topenetrate into metal substrate 10. The method of ion implantationincludes accelerating ions using high energy to cause the ions tocollide with and penetrate into the surface of metal substrate 10.

Carbon diffusion-inhibiting ions 21 may be any ions capable ofinhibiting the diffusion of carbon atoms 31 by blocking the spacesbetween metal atoms 11. In an example embodiment, carbondiffusion-inhibiting ions are ions having an atomic number of less thanor equal to 20. In a further example embodiment, carbondiffusion-inhibiting ion 21 is a nitrogen ion or a boron ion.

When plasma is used to form ion penetration layer 30, the ion-forminggas is a gas including carbon diffusion-inhibiting ion 21. For example,the ion-forming gas may include nitrogen gas (N₂), borane gas (B₂H₆),etc. Plasma implantation may be performed under conditions where the ionforming gas is present in an amount greater than or equal to about 10volume %. When the amount of ion-forming gas is insufficient, carbondiffusion-inhibiting ions 21 may not sufficiently penetrate into metalsubstrate 10. The balance of the gas used in the plasma implantationprocess may be hydrogen gas.

Formation of on penetration layer 20 may be performed at a temperatureof about 300 to about 550 degrees Celsius. When the temperature is toolow, carbon diffusion-inhibiting ions 21 may not sufficiently penetratemetal substrate 10. When the temperature is too high, the metal in metalsubstrate 10 may react with carbon diffusion-inhibiting ions 21,resulting in diminished corrosion characteristics and increased contactresistance. For example, if chromium in metal substrate 10 reacts withnitrogen as carbon diffusion-inhibiting ion 21 to form a chromiumnitride (CrN, Cr₂N), the corrosion resistance of the fuel cell separatordegrades, and the contact resistance increases.

In a further example embodiment, ion penetration layer 20 is formed at alow temperature, which is different from a nitriding process of addingnitrogen at a high temperature.

The process of forming ion penetration layer 20 may last from about 10to about 120 minutes. If the formation time is too short, carbondiffusion-inhibiting ions 21 may not sufficiently penetrate metalsubstrate 10. Even when the formation time is prolonged, the amount ofcarbon diffusion-inhibiting ion 21 penetrating metal substrate 10 islimited.

Next, a carbon coating layer 30 is formed on ion penetration layer 20.Carbon coating layer 30 may be formed by methods includingplasma-enhanced chemical vapor deposition (“PECVD”). Specifically, themethod includes using plasma to excite carbon atoms in a carbonprecursor gas to provide carbon coating layer 30.

The carbon coating layer formation method may be performed in anatmosphere including about 3 volume % to about 30 volume % of a carbonprecursor gas. When the amount of the carbon precursor gas is too low,carbon coating layer 30 may not properly form on metal substrate 10.When the amount of the carbon precursor gas is too high, an increase inresidual stress in carbon coating layer 30 may lead to delamination ofcarbon coating layer 30. In a further example embodiment, the carboncoating layer formation method may be performed under an atmosphereincluding about 10 volume % to about 20 volume % of a carbon precursorgas. The carbon precursor gas can be any capable of exciting a carbonatom by applying plasma. Specifically, the carbon precursor gas may be ahydrocarbon gas. More specifically, the carbon precursor gas may includeC₂H₂, CH₄, or C₂H₆. The balance of the gas used in the process otherthan the carbon precursor gas may be argon or hydrogen gas.

Formation of carbon coating layer 30 may be performed at a temperatureof from about 300 to about 550° C. When the temperature is too low,carbon coating layer 30 may not properly form on the surface of metalsubstrate 10. When the temperature is too high, a chromium carbide(Cr₇C₃) is formed, resulting in lower corrosion resistance.

The example embodiments of a fuel cell separator described above haveimproved corrosion resistance and conductivity, and are therefore usefulfor application in a fuel cell.

Hereinafter, examples of the present invention and comparative examplesare described. These examples, however, should not in any sense beinterpreted as limiting the scope of the present invention.

Example 1

SUS 316L, an austenite-based stainless steel, was used as metalsubstrate. The metal substrate was heated to 300° C. under an argonatmosphere and then contacted with plasma to remove an oxide film formedon the surface of the metal substrate. Next, the metal substrate washeated to 400° C. The argon atmospheric gas was replaced with 15 volume% of nitrogen and 85 volume % of hydrogen, the metal substrate wascontacted with plasma for 10 minutes to allow nitrogen ion to penetratethe metal substrate. The atmospheric gas was again replaced with 20volume % of acetylene (C₂H₂) and 80 volume % of hydrogen, and the metalsubstrate was contacted with plasma for 30 minutes to generate a carboncoating layer.

The resulting fuel cell separator had a thickness of 500 nm.

FIG. 4 is a scanning electron microscopic (SEM) image showing a surfaceof the carbon coating layer of the fuel cell separator obtained fromExample 1. FIG. 4 confirms that the process formed a dense carboncoating layer on the entire surface of the metal substrate.

FIG. 5 shows the results of a GD-OES of the fuel cell separatoraccording to Example 1. As shown in FIG. 5, a carbon coating layerincluding greater than or equal to 70 wt % of carbon was formed to adepth of 20 nm, and beneath the carbon coating layer, an ion penetrationlayer including 5 wt % to 30 wt % of nitrogen was formed to a depth of100 nm.

The density of the carbon coating layer was determined to be 2.68 g/cm3.

Example 2

Example 2 was prepared according to the same procedure as in Example 1,except that the nitrogen ion was allowed to penetrate the metalsubstrate for 20 minutes during formation of the ion penetration layer.

Example 3

Example 3 was prepared according to the same procedure as in Example 1,except that the nitrogen ion was allowed to penetrate the metalsubstrate for 30 minutes during formation of the ion penetration layer.

Example 4

Example 4 was prepared according to with the same procedure as inExample 1, except that the nitrogen ion was allowed to penetrate themetal substrate for 40 minutes during formation of the ion penetrationlayer.

Comparative Example

SUS 316L, an austenite-based stainless steel, was used as the metalsubstrate. The metal substrate was heated to 300° C. under an argonatmosphere and then contacted with plasma to remove an oxide film formedon a surface of the metal substrate. The argon atmospheric gas wasreplaced with 20 volume % of acetylene (C₂H₂) and 80 volume % ofhydrogen, and the metal substrate was contacted with plasma for 30minutes to generate a carbon coating layer.

The resulting fuel cell separator had a thickness of 500 nm.

FIG. 6 is a scanning electron microscopic (SEM) image showing a surfaceof the carbon coating layer of the fuel cell separator obtained fromComparative Example 1. As shown in FIG. 6, the carbon coating layerformed in islands rather than as a dense carbon coating layer.

Experimental Example 1: Evaluation of Adhesion Strength

Adhesion strength was evaluated while increasing pressure up to 50 N foreach fuel cell separator obtained from Example 1 and ComparativeExample, and the results are shown in FIG. 7 and FIG. 8.

As shown in FIG. 7, delamination did not occur in the fuel cellseparator of from Example 1 even when pressure was increased to 50N. Incontrast, as shown in FIG. 8, delamination occurred in the fuel cellseparator of the Comparative Example when the pressure exceeded 8.2N(white line). The white spot indicates the metal substrate in thedrawing.

Experimental Example 2: Evaluation of Contact Resistance and CorrosionCurrent Density

A contact resistance and a corrosion current density were measured foreach fuel cell separator obtained from Examples 1 to 4 and ComparativeExample, and the results are shown in FIGS. 9 and 10.

Contact resistance was measured using a contact resistance tester afterdisposing the separator and the gas diffusion layer between two copperplates and compressing them at 10 kgf/cm².

Corrosion current density was measured using a 0.1N H₂SO₄+2 ppm HFsolution. The solution was heated at 65 degrees Celsius and bubbled withair for 1 hour, and then it was measured within a −0.25 to 1V vs SCErange, and the physical properties were compared and evaluated using thedata of the cathode environment (0.6V vs SCE).

As shown in FIG. 9 and FIG. 10, in a case of the Comparative Examplehaving no ion penetration layer, both contact resistance and thecorrosion characteristics were inferior to those of Examples 1 to 4.

As also shown in FIGS. 9 and 10, as the time of formation of the ionpenetration layer increased, contact resistance decreased and the fuelcell exhibited improved corrosion characteristics.

While this invention has been described in connection with practicalexample embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments, but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claims. Therefore, theembodiments described above should be understood to be examples but notlimiting the present invention in any way.

DESCRIPTION OF SYMBOLS

-   -   100: fuel cell separator 10: metal substrate    -   11: metal atom 20: ion penetration layer    -   21: carbon diffusion-inhibiting ion 30: carbon coating layer    -   31: carbon atom 32: carbon-carbon bond

What is claimed is:
 1. A fuel cell separator, comprising a metalsubstrate having a surface; an ion penetration layer including carbondiffusion-inhibiting ions extending from the surface of the metalsubstrate into the metal substrate; and a carbon coating layer disposedon the surface of the metal substrate; wherein the ion penetration layerincludes from 5 wt % to 30 wt % of the carbon diffusion-inhibiting ionand from 5 wt % to 85 wt % of carbon.
 2. The fuel cell separator ofclaim 1, wherein the carbon diffusion-inhibiting ions of the ionpenetration layer are interposed between metal atoms in the metalsubstrate.
 3. The fuel cell separator of claim 1, wherein the carbondiffusion-inhibiting ion is a nitrogen ion or a boron ion.
 4. The fuelcell separator of claim 1, wherein the ion penetration layer has athickness of from about 30 nm to about 300 nm.
 5. The fuel cellseparator of claim 1, wherein the carbon coating layer has a thicknessof from 10 nm to 1000 nm.
 6. The fuel cell separator of claim 1, whereinthe carbon coating layer has a density of from 2.0 g/cm³ to 3.0 g/cm³.